Bergeron also cautions against viewing efficient training as a fitness cure-all. Doing hard intervals for 30 minutes in the morning will do little to combat the deleterious effects of an inactive or sedentary lifestyle the rest of the day, he said.

Runners mustn't forget to address full-body conditioning, including cardio, flexibility, range of motion and weight training to protect connective tissue, said William Kraemer, professor in the University of Connecticut's Department of Kinesiology. As evidenced in the runners of the 1960s and '70s — who today face myriad orthopedic challenges — doing the same thing every day breaks a body down, he said.

Kraemer also encourages a reality check: Not every body is made to run a marathon. And while training shorter and smarter can have good results, he cautions against treating it as a shortcut.

"We live in a world where everyone wants to get in shape in a week," Kraemer said.

For Ancona, whose dedication has led him to win several triathlons and be named male athlete of the month in last year's Chicago Athlete magazine, the new training regimen has made a big difference in his life and performance.

He has cut his training time down to 10 to 15 hours weekly and hasn't suffered any injuries. He also has shaved 41 minutes off his Ironman distance race time, from about 9 hours, 50 minutes in 2010 to 9 hours, 9 minutes last year.

Ancona's training schedule looks something like this: He bikes three days a week, runs four days a week and swims three days a week, alternating long and easy training days with short, hard interval days, and does nothing on Mondays. Every six to eight weeks Skiba re-evaluates him, aiming to push the threshold up.

Even without such calculations, recreational runners can reap similar benefits by following a targeted plan and making sure not to overdo it.

"The biggest thing (Skiba) does is hold me back in training, so that I can perform better in races," Ancona said.